Nitrosomethane, and

For the first time, the primary nitrone (formaldonitrone) generation and the comparative quantum chemical analysis of its relative stability by comparison with isomers (formaldoxime, nitrosomethane and oxaziridine) has been described (357). Both, experimental and theoretical data clearly show that the formal-donitrones, formed in the course of collision by electronic transfer, can hardly be molecularly isomerized into other [C,H3,N,0] molecules. Methods of quantum chemistry and molecular dynamics have made it possible to study the reactions of nitrone rearrangement into amides through the formation of oxaziridines (358). 

C-nitroso compounds exhibiting an a-H, such as nitrosomethane and nitrosoethane, may isomerize to the corresponding oxime, as evidenced by mass spectrometry116,117. Unsaturated C-nitroso compounds may undergo electrocyclic ring closure as discussed below. 

The authors of primary Reference 80 present their own and selected literature values for the R—NO bond enthalpies for the hydrocarbyl cases of Me, Et, t-Bu, allyl and benzyl, as well as mixed fluorinated, chlorinated methyl radicals. We now wish to compare nitroso species with the corresponding amino and nitro compounds. Choosing what we consider the most reliable and relevant nitroso compound data, and accompanying them with the corresponding radical data, we derive enthalpies of formation of gaseous nitrosomethane, 2-methyl-2-nitrosopropane and o -nitrosotoluene81 to be 65 2, —29 4 and 174 7 kJmol-1. (By comparison, the earlier values recommended8 for nitrosomethane and 2-methyl-2-nitrosopropane were 70 and —42 kJmol-1 respectively.)... 

A related procedure, which may be of value from the preparative standpoint, involves the preparation of /rans-nitrosomethane dimer by adding a solution of diacetyl peroxide in sec-butyl nitrite to warm sec-butyl nitrite [50]. From the product of the reaction it has been assumed that this preparation involves the generation of free methyl radicals which react with the nitrite to give nitrosomethane and alkoxy radicals. The latter disproportionate to ketones and alcohols, while the nitroso compound dimerizes. 

The thermal cis-trans isomerizations of nitrosomethane and 2-methyl-l-nitroso-propane obey first order kinetics with activation energies (kcal.mole ) and log y4(sec ) of 19.1 and 22.1, and 11.6 and 13.9, respectively . Both direct and indirect isomerization mechanisms were considered, viz. 

Elastomeric terpolymer fi om tetrafiuoroethylene, trifluoro-nitrosomethane, and a small amount of an unsaturated monomer, e.g., nitrosoperfluorobutyric acid and nitroso or carboxy nitroso rubber... 

Liittke has studied the changes in the spectrum which occur with time when primary and secondary nitroso-compounds are volatilised. In the cases of nitrosomethane and nitrosocyc/ohexane, for example, absorptions appear at 1564 cm and 1558 cm which can be associated with the —N=0 frequency. These bands rapidly diminish in intensity on standing and new bands appear, so that in a relatively... 

Fluoronitrosopolymers have potential primarily as low temperature, oil-resistant elastomers but although they have attracted a good deal of interest, these materials do not appear to have progressed beyond the pilot plant stage. The most thoroughly investigated nitrosopolymer is the copolymer of trifluoro-nitrosomethane and tetrafluoroethylene [4]. 

Nitrosomethane and formaldoxime both have the molecular formula CH3NO and the connectivity CNO. All of the hydrogens are bonded to carbon in nitrosomethane. In formaldoxime, two of the hydrogens are bonded to carbon and one to oxygen. Write Lewis formulas for (a) nitrosomethane and (b) formaldoxime that satisfy the octet rule and are free of charge separation. 

Arden, E.A., and L. Phillips (1964), Reactions in the diethyl peroxide-nitric oxide system. Part I. Oxidation and reduction of nitric oxide Reactions of nitrosomethane and nitroxyl, J. Chem. Soc., 1964, 5118-5125. 

Spectroscopic Data. Freshly prepd nitrosomethane has an IR absorption band at 6.3—6.4 p which slowly decays and is replaced by a band at 11 y characteristic of formaldehyde oxime (Ref 8). The visible absorption Amax is 287my in eth and at 266—7m p in w (Ref 4)... 

The Hel PE spectrum of nitrosomethane (Figure 13) was first studied by Bergmann and Bock125,127. This compound as well as several other aliphatic and aromatic C-nitroso compounds were investigated by Pfab and coworkers126,128, however several of them were dimers. 

FIGURE 13. PE spectra of nitrosomethane (b) and its cfr-dimer (a) displaying the temperature-dependent decomposition of cw-(MeNO)2 into MeNO. Reproduced (modified) with permission from Reference 125... 

TABLE 12. Ionization potentials IP (eV) and orbital energies s (eV) of nitrosomethane... 

In Table 13 the ionization potentials of some more C-nitroso compounds are collected. The spectrum of monomeric t-nitrosobutane126 exhibits a well separated band at 9.05 eV. The following ionizations show maxima at 11.85 and 12.46 eV. The spectrum is dominated by a strong composite band from 12.9-14.5 eV. The spectrum can be assigned by comparison with nitrosomethane. The substitution of Me by t-Bu lowers the n ionization energy of the nitroso group by 0.7 eV, whereas the n+ and it ionization energies are lowered by 1.8 and 1.7 eV, respectively. 

The photoelectron spectra of nitrosomethane, 2-methyl-2-nitrosopropane and perhalogeno nitro someth anes has been re-examined and re-assigned on the basis of ab initio SCF-CI calculations139. Photoionization quantum yields140 have been measured for 2-methyl-2-nitrosopropane at wavelengths 147, 123, 105 and 107 nm. The results show that photoionization at energies up to 1.5 eV above threshold is of low probability. The data have been compared with those of recent photoelectron spectroscopy. 

The quintessential C-nitroso compound nitrosomethane, CH3-NO (1), was first isolated by Coe and Doumani in 1948 while already in 1936 Staveley and Hinshelwood found that addition of small quantities of nitric oxide to the reaction vessel during the... 

SCHEME 1. Isomers of nitrosomethane. Besides 4 and 5 deprotonation of 1, 2 and 3 would lead to the formation of a nitrosomethanide, while in case of 4 and 5 only an isomerization process following the deprotonation can give nitrosomethanide... 

While highly reactive formaldonitrone (3) was detected only recently, both the nitrosomethane (1) and the formaldehyde oxime (2) are also reactive compounds in the condensed phase 2 undergoes facile oligomerization , whereas 1 forms a dimer and isomerizes to 2 . Deprotonation of any of these isomers (1, 2, 3) would result in the corresponding simplest nitrosomethanide anion [H2C=N—O]. ... 

Nitrosomethane (1) is known to be less stable than its isomer formaldoxime 2 and original attempts to isolate this species failed owing to its facile isomerization to the oxime 2. Already Bamberger and Seligman considered in 1903 that it would be difficult to isolate nitrosomethane after oxidation of methylamine due to its rapid isomerization to 2. Hence, 2 is always present in the synthesis of the nitrosomethane. Nitrosomethane is produced in the pyrolysis or photolysis of tcrf-butyl nitrite and by the reaction of methyl radicals with nitric oxide. Early results were confusing since the final product obtained is dimeric nitrosomethane. It was first isolated in 1948 by Coe and Doumani from the photolysis of gaseous ferf-butyl nitrite according to the overall reaction shown in equation 2. 

Batt, Gowenlock and Trotman carried out a detailed study of the pyrolysis and photolysis of terf-butyl nitrite and established that dimeric nitrosomethane exists in two isomeric forms, cis and trans (Scheme 5). Monomeric nitrosomethane could be generated by heating the dimer in the gas phase (the activation energy for dissociation was found to be ca 90 kJmoH )". Also ultraviolet irradiation dissociates the dimer, leaving monomeric 1. Vibrational analysis of monomeric 1 is summarized in Table 4. 

TABLE 4. Experimental and calculated vibrational frequencies (cm ) of monomeric nitrosomethane, IR intensities in parentheses (kmmoR )... 

For the photolysis of ferf-butyl nitrite a possible reaction mechanism (Scheme 6) consists of the production of ferf-butoxy radicals (equation 3), followed by their decomposition to give acetone and methyl radicals (equation 4). The latter are trapped by the nitric oxide liberated in the first step (equation 5). However, the absence of ethane production in the actual experiments suggested that an intramolecular formation of nitrosomethane is unlikely ". ... 

Formaldonitrone, CH2=N(H)—O (3), the elusive simplest organic nitrone, has been prepared transiently in the gas phase by femtosecond collisional neutralization of its cation radical, CH2—N(H)—0+". The latter was generated by dissociative ionization of 1,2-oxazolidine. Nitrone 3 showed negligible dissociation upon collisional neutralization and was distinguished from its tautomers formaldoxime 2 and nitrosomethane 1 that gave different NR mass spectra. The enthalpy of formation was calculated from enthalpies of atomization and two isodesmic reactions as Af//29s(3) = 58 1 kJmol . The calculated, large activation barriers for isomerization of 3 (179 and 212 kJmoH for 3 anti-2 and 3 1, respectivelyindicate that once 3 is formed and diluted in the gas phase it should not isomerize unimolecularly to either 1 or (syn/anti) 2. 

The dimer of nitrosomethane (azo-dioxide), (CHsNO) , exists at ambient temperatures in two isomeric forms, cis and trans (Figure 8), which undergo isomerization relatively easily. rrawi-(CH3—NO)2, having a planar azo-dioxide moiety, crystallizes in the orthorhombic space group Cmcm with Z = 4. The experimentally determined bond lengths are rf(N—O) = 1.25 and rf(N—C) = 1.57 A, respectively (calculated data, see Figure 8). ... 

As in other areas of the chemistry of nitroso compounds, the tacit assumption has been that the rearrangement of aliphatic nitroso compounds bearing hydrogens on the carbon atom which also carries the nitroso group rearranges to the oxime virtually instantaneously. Yet, recently, the existence of all types of aliphatic nitroso compounds has been well established. Without going into details here, nitrosomethane, nitrosocyclohexane, and nitrosodibenzoyl-methane have been prepared and appear to exist as reasonably stable dimers. 

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